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Plan-Specific Dose Calibration for IMRT QA

W Luo

W Luo*, Y Meng , University of Kentucky, Lexington, KY


SU-I-GPD-T-278 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the effect of dose calibration on IMRT QA passing rate, and find an optimal dose calibration method to improve IMRT QA accuracy and passing rate.

Methods: IMRT QA dose measurement with a phantom is calibrated using a calibration factor (CF) that is the ratio between the expected value and measured value that corresponds to the reference fields delivered on the phantom. The PTW Octavius phantom was used for this study. CFs were derived for the dose at the isocenter using the following beam arrangements on different dates: AP field with field sizes ranging from 2x2 to 20x20 cm2, lateral field, and 4-Field – 4-beam box. Plan-specific CFs were also derived for the same beam geometry as the patient plans in two ways: 100 MU for each beam (Plan beams) and MUs used for the patient plans (Plan MU). Both static IMRT plans and RapidArc plans were included in this study. The results were evaluated with PTW-VeriSoft (Gamma criteria of 3%/3mm).

Results: CFs for AP beams ranged from 1.024 to 1.039 with the field size ranging from 2x2 to 20x20, while for 4-beam box, it ranged from 1.021 to 1.053 for the same field sizes. Applying those CF values to static IMRT QA plans, the Gamma index passing rate changed by up to 15.6% for 3%/3mm, and 7.9% for 2%/2mm. In the case of RapidArc plans, the passing rate was very high for all different CFs, and changed by up to 0.8% for 3%/3mm, and 4.7% for 2%/2mm. The use of plan-specific calibration increased the passing rate by up to 24.9% for 3%/3mm for static IMRT QA, and 12.7% for 2%/2mm.

Conclusion: IMRT QA passing rates are significantly correlated with dose calibration methods used. The plan-specific dose calibration can be used to improve IMRT QA passing rate.

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